A situation is assumed here in which a catalytic converter in the exhaust gas line to an internal combustion engine has the capacity to store oxygen. In this way the emissions of the system are reduced. If the internal combustion engine releases carbon monoxide to an increased degree, it is oxidized into carbon dioxide with oxygen from the catalytic converter. If the internal combustion engine releases nitrogen oxides to an increased degree, they are reduced, and the remaining oxygen is stored in the oxygen storage system. It is assumed that the capacity to hold and release oxygen is not exhausted. So that this is the case at any time, regulation takes place using a lambda probe which is located downstream of the catalytic converter or at least downstream of a section thereof.
Regulation stands and falls with the functional reliability of this lambda probe. Reliably determining the oxygen storage capacity of the catalytic converter is possible only when the lambda probe is fully functional. To determine the oxygen storage capacity, the exhaust gas line is supplied in alternation with lean and rich exhaust gas. In the process, an almost completely oxygen-free catalytic converter is gradually charged with oxygen until it is almost completely charged; then again the oxygen is almost completely eliminated, etc. Conclusions about the oxygen storage capacity can be drawn on the basis of the signals of the lambda probe. The oxygen storage capacity is determined here based on the measurement of the interval between the change from rich to lean exhaust gas or vice versa on the one hand and the signals of the lambda probe passing through a threshold value on the other hand. If the lambda probe is not completely functional, i.e., aged or poisoned, this threshold value is, however, crossed only later. This could result in an increased oxygen storage capacity being indicated erroneously. Failure to detect a lambda probe that is not fully functional and an oxygen storage system that is not fully functional at the same time should be avoided, or a fully functional oxygen storage system could be erroneously diagnosed by the impaired probe.
The diagnosis of the oxygen storage capacity of the catalytic converter therefore must be preceded by a diagnosis of the functional reliability of the lambda probe.
In DE 2008 023 893, it is described that at least one signal can be used for diagnosis which has been recorded in time by the lambda probe when changing from a lean to a rich exhaust gas or vice versa. It is not necessary to wait for a time interval after the change, but signals that are assigned directly to this change can then be used. The expression “in time when” can include first waiting for a delay time to expire. The delay time begins with the detection of the change by another lambda probe located upstream of the catalytic converter and can extend over 20 to 50 ms. After the delay time expires, signals in a further time interval are then used for diagnosis.
The method from DE 10 2008 023 893 works reliably in most of the settings of the control device which controls the delivery of fuel to the internal combustion engine and therefore the change from lean to rich exhaust gas.
To date there has not been a teaching regarding how it can be induced that under all possible conditions the method for diagnosis of the functional reliability of a lambda probe will work in each instance, using signals recorded by this lambda probe, when changing from rich to lean exhaust gas or vice versa.
Thus, the object of the invention is to make the method from DE 10 2008 023 893 more reliable, i.e., to have it proceed without problems under all possible conditions, even unusual conditions.